1. Field of the Invention
The present invention relates to an optical module and particularly to an optical module having a function for optically coupling an optical fiber and a lens to each other.
2. Related Art
In optical communication using optical fiber, transmission capacity thereof has increased rapidly in recent years. For example, in a relay station, use of a large number of optical fibers in parallel to one another has become frequent with the advance of laying of optical fiber close to users such as business users and home users. Because it is necessary to insert various optical elements in transmission paths based on these optical fibers, it is necessary to couple the large number of optical fibers to the optical elements collectively. Generally, a lens is used so that light output from each optical fiber is made incident onto a corresponding optical element efficiently or, conversely, light output from each optical element is made incident onto a corresponding optical fiber efficiently.
Particularly when such a large number of optical fibers as described above are to be coupled, provision of coupling portions as a module is preferable for miniaturization of the device and improvement of reliability. An optical module using a planar microlens array having a plurality of very small lenses (microlenses) formed on a flatplate-like transparent substrate has been developed for this purpose. Use of such an optical module permits the large number of optical fibers to be efficiently coupled to the optical elements.
Incidentally, the numerical aperture (NA) of a silica-based single mode optical fiber is about 0.1 in air on the assumption that the refractive index difference between a core and a clad is from 0.2% to 0.3%. When a spacer made of a material (such as glass) having a refractive index higher than that of air is inserted between a lens and an optical fiber, the NA with respect to light output from the optical fiber is reduced. When, for example, the spacer is made of glass having a refractive index n=1.46, the NA is reduced to about 0.069.
Generally, if a silica-based single mode optical fiber 10 is provided so as to be far by a certain distance or longer from a lens 22 when light 16 output from the optical fiber 10 is to be coupled to the lens 22 or lens array as shown in FIG. 7A, a part 18 of the light 16 output from the optical fiber 10 is left (kicked) out of the lens area as shown in FIG. 7B. As a result, there arises a problem that coupling loss increases.
Generally, because the effective area of a lens is a center portion which is 90% as large as the total area of the lens, light incident onto the other area of the lens is kicked out so that coupling efficiency is worsened. Furthermore, in the case of a lens array having lens elements arranged closely as shown in FIGS. 7A and 7B, the light kicked out is input into adjacent lenses to bring deterioration of crosstalk as well as the light is kicked out. Accordingly, in order to keep coupling efficiency and crosstalk to a certain degree or higher, the optical fiber-lens distance must be reduced to be not larger than a certain value or a lens having a large diameter must be used. As a result, there arises a problem that a limit occurs in optical design because the optical fiber-lens distance is limited or that the optical module is large-sized because a lens having such a large diameter is used.
Processing a tip of each optical fiber into a hemispherical tip by electric discharge machining, polishing, etching or the like is known as a method for improving such optical fiber-lens coupling efficiency. The tip of the optical fiber is processed to have a lens function to suppress spread of output light to thereby reduce both kicking-out of light and closstalk. The degree of freedom for the obtained lens effect is however small because this processing is provided for deforming the shape of the tip of the optical fiber. Furthermore, complex steps are required to cause more labor and cost because the hemispherically ending process must be added.